In general, magnetic resonance imaging (MRI) examinations are based on the interactions among a primary magnetic field, a radiofrequency (RF) magnetic field, and time varying magnetic gradient fields with a gyromagnetic material having nuclear spins within a subject of interest, such as a patient. Certain gyromagnetic materials, such as hydrogen nuclei in water molecules, have characteristic behaviors in response to external magnetic fields. The precession of spins of these nuclei can be influenced by manipulation of the fields to produce RF signals that can be detected, processed, and used to reconstruct a useful image.
The magnetic fields used to generate images in MRI systems include a highly uniform, static magnetic field that is produced by a primary magnet. A series of gradient fields are produced by a set of gradient coils located around the imaging volume in which the subject is placed. The gradient fields encode positions of individual plane or volume elements (pixels or voxels) in two or three dimensions. An RF coil is employed to produce an RF magnetic field. This RF magnetic field perturbs the spins of some of the gyromagnetic nuclei from their equilibrium directions, causing the spins to precess around the axis of their equilibrium magnetization. During this precession, RF fields are emitted by the spinning, precessing nuclei and are detected by either the same transmitting RF coil, or by one or more separate coils. These signals are amplified, filtered, and digitized. The digitized signals are then processed using one or more algorithms to reconstruct a useful image.
While MRI images provide a variety of benefits and may be particularly useful for certain imaging contexts, such images may be less useful in other contexts. By way of example, while MRI images provide good spatial resolution and anatomical soft tissue contrast, such images may still be limited in their usefulness for detecting and delineating cancer lesions at an early stage, i.e., when the lesions are most easily cured and treated. That is, MRI images generated using current approaches may offer insufficient delineation of tumor and tissue boundaries for diagnostic and therapy purposes and may be unsuitable for characterization of salient characteristics for cancer diagnosis (e.g., characterization of a lesion as malignant or benign).
Conventional approaches at solving this problem have involved the introduction of contrast media to the patient. Such contrast media are based on large molecular chelates of paramagnetic ions such as Gadolinium (Gd), Manganese (Mn), or on hyperpolarized Carbon-13. Such contrast enhancement approaches, however, are invasive, involving the intravenous injection of an exogenous contrast media to the patient. As discussed herein, an MRI contrast enhancement approach that does not involve the administration of exogenous compounds is desirable.